A sensor for wetness condition of diapers is of great benefit to a caretaker of a baby as it relieves the caretaker from the frequent task of checking on the baby. It is of great benefit to the baby as the immediate attention of the caretaker prevents the condition known as "diaper rash". A sensor that remotely calls the attention of the caretaker is of obvious added benefit. Wetness sensors with remote alarm have been suggested by several authors. For example, Kline et al. in U.S. Pat. No. 5,392,032 uses a pair of electrodes as sensors and a detachable electronic circuit to power an alarm. It will be prohibitively expensive to implant the electronic circuit in a disposable diaper. Since disposable diapers are preferred by many, an implantable sensor of low cost has an advantage over that described in the above Patent for two main reasons: 1) a detachable unit must be large (large enough so it cannot be swallowed by the baby in case it detaches or left near the baby by neglect)--a large attachment will cause discomfort and 2) caretakers would not like to handle a soiled diaper for the purpose of detaching the reusable unit.
The present invention provides a cost effective remote wetness sensor for disposable diapers. The invention borrows an idea from theft prevention systems that utilize the so called Passive Tag. Such a system is described by Cole et. al in U.S. Pat. No. 3,707,711. A tag, or in the present invention an implant in the diaper interferes with the transmission of an external transmitter only upon a condition of wet diaper. This interference is detected by a receiver also external to the diaper. Since with early detection of wetness the amount of absorbent filler in the diaper can be reduced, it is possible that with the present invention the total cost, over time, will actually be lower.